Other than reading your interviewer's body language or coming right out and asking if he or she thinks you're fit for the job, one simple question may be all you need to gauge how an interview is going or went: "Why do you work here?"

Steve Browne, an HR director, says on HR Bartender that the way the person answers can be a sign that the interview went well (or not):

I recommend that people ask "Why do you work here?" as their first follow-up question every time. When they do this, the interviewer will inevitably let their guard down and the candidate can judge how the person answers. If they seem to relax and engage the candidate, the interview went well. If they are aloof and give the Company "line" as a response, then it may have not gone well.